In recent years, the density of magnetic disc devices or the like has increased. To accommodate such higher densities, there is a need for a magnetic recording medium (hereinafter sometimes referred to as a "medium") suitable for a magnetic head (hereinafter referred to as a "MR head") using a high magnetic resistance effect that is capable of high reproduction sensitivity.
The MR head has low head noise as compared with conventional magnetoelectric induction type heads. Therefore, the reduction of medium noise has become very important for improving the S/N of the device as a whole.
In a magnetic recording disc for a hard disc device (HDD) commonly used at present, a non-magnetic underlayer (e.g., Cr, Cr alloy), a magnetic layer (e.g., CoCrTa, CoCrPtTa alloy) and a protective layer (e.g., carbon) are provided in sequence on a non-magnetic substrate. Also, a lubricating layer comprising a liquid lubricant is formed thereon.
In order to optimize the recording/reproducing properties of the above-described magnetic recording medium, JP-B-5-23564 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes a Cr underlayer having a thickness of from 50 to 200 .ANG. in order to increase squareness.
Furthermore, JP-A-1-232522 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes to add one or more metals selected from Cu, Nb, Ti, V, Zr, Mo, Zn, W and Ta to the Cr underlayer in order to enhance the magnetic properties, and particularly coercive force.
Furthermore, EP 0,704,839A1 proposes to use an alloy having a B2 ordered structure (e.g., AlNi, AlCo, AlFe) as the underlayer to reduce the crystal size of the magnetic layer and to thereby provide improved noise characteristics.
When the Cr underlayer is formed having a thickness of from 50 to 200 .ANG. as proposed in JP-B-5-23564, the Cr crystal structure is not oriented toward the (200) plane. As a result, sufficiently high coercivity cannot be obtained and the medium is not suitable for a MR head.
In JP-A-1-232522, the thickness of the Cr underlayer is from 500 to 3,000 .ANG. such that the Cr grains can be oriented toward the (200 plane). Consequently, the noise can be reduced. Furthermore, although Cr or a Cr alloy is used as the non-magnetic underlayer of the smoothened substrate, problems arise in that the anisotropy controlling effect of the disc by texturing or the like is lowered to cause a reduction in coercivity and in turn a decrease in the reproduction output.
When an Al alloy having a B2 ordered structure (e.g., AlNi, AlCo, AlFe, etc.) is used as an underlayer, as proposed in EP 0,704,839A1, the crystal size of the magnetic layer is reduced due to the Al--Ni alloy or Al--Co alloy and the noise is improved. However, it is difficult to increase coercivity using the Al--Ni alloy or to increase coercive squareness using the Al--Co alloy. Accordingly, the reproduction output is small. Thus, a problem still remains in performing high density recording.